Mould tools for plastic materials are made of a great number of various steel alloys, including martensitic, medium alloyed steels. In that group there is a commercially available steel which nominally contains 0.6% C, 4.5% Cr, 0.5% Mo and 0.2% V and which is used for cold work tools and mould tools for plastic materials. Within the same group there is also found the standardised steel AISI S7 which is also sometimes used for inter alia mould tools for moulding plastic materials, and another commercial available tool steel, which nominally contains 0.55% C, 2.6% Cr, 2.25% Mo and 0.9% V. The two first named steels attained a desired hardness only after low temperature tempering, which may cause risk for retained tensions in the steel after heat treatment. It is true that the last mentioned steel may achieve an adequate hardness after high temperature tempering, i.e. tempering at about 550° C., on the other hand the hardenability of that steel is not particularly good.
It is the purpose of the invention to provide a mould steel for moulding plastic materials which has a better combination of features for the employment of the steel for the manufacturing of mould tools for plastic materials, than the tool steels which presently are commercially available. Particularly, the steel should have the following features:                Good ductility/toughness,        Good hardenability allowing through hardening in connection with conventional hardening in a vacuum furnace of products with thicknesses up to at least 350 mm,        Adequate hardness, at least 54 HRC, preferably at least 56 HRC, after hardening and high temperature tempering, which gives a high resistance against plastic deformation and, at least as far as certain applications are concerned, also an adequate wear resistance without nitriding or surface coating with titanium carbide and/or titanium nitride or the like by means of e.g. PVD- or CVD-technique,        Good tempering resistance in order to allow nitriding or surface coating with titanium carbide and/or titanium nitride or the like by e.g. any of said techniques without reduction of the hardness of the material for applications which require particularly good wear resistance of the tool,        Good heat treatment features,        Good grindability, machinability by cutting operations, spark machinability, and polishability.        
Other important product features are:                Good dimension stability during heat treatment,        Long fatigue life.        
Specifically, the invention aims at providing a matrix steel which can be employed as a material for mould tools for plastic materials, i.e. a steel which is essentially void of primary carbides and which in its use condition has a matrix consisting of tempered martensite.